Better title group DMs in Android notifications

[rajveermalviya]

In group DMs, the Android notification's conversation title currently displays only the latest sender's display name, such as "from Greg Price with 1 other".

Instead, it should name the other recipients too, up to 3 names (and excluding self). Beyond that, it should name 3 of the members of the thread and use the "N others" form for the rest, as in "Alice, Bob, Carol, and 2 others".

The difficulty here is that while for the message's sender, the server includes their display name (and avatar URL), for the other recipients we have just their user IDs but no details like display names. There are two options for how we could handle this:

• We could have the server start including it in notification payloads. That'd start working promptly for users on Zulip Cloud and other servers regularly updated from Git. We'd fall back to the current text when it's not available.
  • This isn't ideal because it involves denormalizing more and more data into the notification payloads. But it would be easy to do.
• We could start keeping the list of users, including their names, in the database so that we have it ready when a notification arrives.
  • We'll want to do this eventually for the sake of #477.
  • This is something we won't do soon, though, because it'll involve adding significant new complexity where we update data on server events. That's worth it for #477, because that's a major improvement to the UX, but is too much for this issue to justify.

Corresponding zulip-mobile issue: https://github.com/zulip/zulip-mobile/issues/5116

[gnprice]

Thanks for filing this! I revised the description, including taking more text from https://github.com/zulip/zulip-mobile/issues/5116 .